A pharmaceutical start-up company has been accused by doctors of withholding data from a clinical trial that revealed the company’s drug increased the risk of heart attack, instead of reducing it, as originally intended.

Anthera Pharmaceuticals, based in Hayward, California, did not release the data to academic investigators, as required, for more than a year, according to a group of cardiologists.

“Despite a contract that required transfer to the academic authors, the company stonewalled every attempt to acquire the data,” Dr. Steven Nissen, a cardiologist at the Cleveland Clinic, said in an email to The New York Times.

Nissen was the senior author of a report on the data published by The Journal of the American Medical Association.

Anthera’s chief medical officer, Dr. Colin Hislop, denied the accusations, saying it simply took time to collect and organize the data. “I don’t think the timeline was particularly protracted, nor were we being difficult,” he told the Times.

Anthera was also required to survey all patients six months after the end of the trial period to see if they were still alive. But the company collected that data for only 31% of the patients, which made it impossible to gauge whether the drug increased the risk of death.

The medication was developed to reduce the risk of heart attacks, stroke, death and chest pains requiring hospitalization.

But the trial was terminated by a safety monitoring committee after early results showed patients who got the drug actually had a higher risk of cardiovascular problems, mainly heart attacks, than those who received a placebo.

Founded in 2004, Anthera is still trying to get its first drug on the market.